Bimodal polymers are a special group of polymeric materials, which, in principle, consist of two intimately mixed polymers of different molecular weight. With polyethylenes of different molecular weight as an example, show the bimodal effect on practical properties and on processing.
Fractions in Polyethylenes (PE), with a molecular weight < 400 g/mol, tend to migrate out of the material and, therefore, affect taste and odor. With molecular weights between 103 and 104 g/mol, the melt flows more readily. The molecules relax quickly and avoid frozen orientations. They also crystallize more easily and uniformly than longer chains. Macromolecules with molecular weights between 10s and 107 g/mol result in higher melt strength and, with increasing chain length, more chains connect the crystallites, thus increasing the toughness and stress cracking resistance of the material.
Bimodal polymers have two molecular weight distribution maxima, one in the low molecular weight region and one in the high molecular weight region, rather than the bell-shaped distribution curve with a single maximum exhibited by conventional, non-bimodal polymers. The macromolecules of different molecular weight can be produced simultaneously by means of catalysts with two reactive centers and thus become completely blended during the polymerization. Another approach is to employ a two step reactor in which the two polymers of different molecular weight are polymerized one after the other. It is also possible to take two compatible polymers, for example polymers that can crystallize together, and to blend them in an extruder.
Fractions in Polyethylenes (PE), with a molecular weight < 400 g/mol, tend to migrate out of the material and, therefore, affect taste and odor. With molecular weights between 103 and 104 g/mol, the melt flows more readily. The molecules relax quickly and avoid frozen orientations. They also crystallize more easily and uniformly than longer chains. Macromolecules with molecular weights between 10s and 107 g/mol result in higher melt strength and, with increasing chain length, more chains connect the crystallites, thus increasing the toughness and stress cracking resistance of the material.
Bimodal polymers have two molecular weight distribution maxima, one in the low molecular weight region and one in the high molecular weight region, rather than the bell-shaped distribution curve with a single maximum exhibited by conventional, non-bimodal polymers. The macromolecules of different molecular weight can be produced simultaneously by means of catalysts with two reactive centers and thus become completely blended during the polymerization. Another approach is to employ a two step reactor in which the two polymers of different molecular weight are polymerized one after the other. It is also possible to take two compatible polymers, for example polymers that can crystallize together, and to blend them in an extruder.
